x86: Display the SPL banner only once

At present on a cold reboot we must reset the CPU to get it to full speed.
With 64-bit U-Boot this happens in SPL. At present we print the banner
before doing this, the end result being that we print the banner twice.
Print the banner a little later (after the CPU is ready) to avoid this.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Tested-by: Bin Meng <bmeng.cn@gmail.com>